COLLISIONAL LOSSES OF FAST PARTICLES IN LOW-CURRENT AXISYMMETRICAL TOKAMAKS

Three-dimensional Fokker-Planck simulation of collisional losses of me ga-electron-volt fusion poducts in axisymmetric tokamaks with plasma c urrents I < 2 MA is carried out. The calculations take into account bo th loss due to radial diffusion and loss caused by pitch-angle scatter ing in the first-orbit loss region in velocity space. Collisional loss es of deuterium-deuterium (D-D) fusion products in the energy range 0. 5 less than or equal to epsilon/epsilon(O) less than or equal to 1 (wh ere epsilon(O) is the birth energy) are found to be increased with pla sma current and comparable to a first-orbit loss at I > 1.5 MA. The lo ss mechanism considered may be responsible for the observed experiment ally delayed losses of D-D fusion products in the Tokamak Fusion Test Reactor (TFTR). The dependencies of collisional losses on plasma curre nt, effective charge number of the plasma (Z(eff)), and aspect ratio a re investigated. The distributions of escaped ions over pitch angles, energies, and poloidal angles are evaluated. The fraction of collision ally lost fast fusion products is shown to scale like (v(p)erpendicula r to/v(s))(0-6) or Z(eff)(0.6) (here v(p)erpendicular to and v(s) are characteristic collision rates of pitch-angle scattering and slowing d own, respectively). The approach used may Be considered as an alternat ive to the approach based on Monte Carlo modeling of scattering and ca n serve as a validity check of the latter.